/Engine/Source/Runtime/Core/Public/Math/BigInt.h
Extended gcd: ax + by = gcd(a, b), where a = exponent, b = fi(n), gcd(a, b) = gcd(e, fi(n)) = 1, fi(n) is the Euler's totient function of n We only care to find d = x, which is our multiplicatve inverse of e (a).
namespace FEncryption { template<typename IntType> IntType FEncryption::CalculateMultiplicativeInverseOfExponent ( IntType Exponent, IntType Totient ) }